Mail Merge is a feature of LibreOffice that allows you to write a template letter with predefined fields and then populate that template with fields from a database. This is very useful if you need to send the same letter to a number of people with only small changes like the name and address of the person you're sending it to.
How to use it
After downloading the mailmerge.odt file (below is the link) and opening it, you can use the macros contained in the document. Edit the template as you see fit and then run the macros using the menus: